Slot ports -- same calculation as round?

[noviygera]

I read that using a slot port with the wall of the cabinet as part of the slot opening needs to be shorter than equivalently modeled round port. Area and length being equal. Is this true and if so, by how much do I need to shorten the slot?

thanks,
Herman

[LOUT]

It depends on the widthXheight ratio...the more "smooshed" that port is shaped, the more drag it'll probably make for the air puffing through it which should allow a slightly shorter port-length to tune the same as a slightly longer port that's technically the same area (heightXwidth) with less drag against the airflow.
This comment is mostly useless since I have NO idea how much shorter the slot-port should be. I don't think the difference is huge, so I think you can be safe by making it roughly the same at the risk of ending up tuned a tiny bit low/deep.
I'm pretty sure there's a nice rule for the ratio differences, but I can't remember it. :(

[noviygera]

Found a page on JL audio that covers this issue. "End correction" paragraph:

https://jlaudio.zendesk.com/hc/en-us/articles/221068047-Enclosure-Ports

The cross-section area of slot should be same as round port. However. End correction formula (simple one) must be applied to calculate the length. As I understand it's a bit shorter than round port because of the long wall being part of the slot port.

Is this in line with your practical experience?

[Chris Roemer]

I made a low-tuned "small" box (< 1cf) for a 10" sub w/quite a long (1.5"x10") slot - over 4ft. It was double/triple folded at the bottom of the box and came within a Hz or 2 of WinISD's "aim" (verified w/a DATS). So close that I wouldn't bother worrying about it.

[billfitzmaurice]

I read that using a slot port with the wall of the cabinet as part of the slot opening needs to be shorter than equivalently modeled round port. Area and length being equal. Is this true

It depends how narrow the slot is. There's a zone of high friction at the wall of all ports, where airflow is restricted. That zone tends to be about an eighth of an inch thick on all the port surfaces. If that zone makes up a considerable portion of the port dimension the area is in effect reduced, be the port a one inch tube or a one inch wide slot. Whether or not the wall of the cabinet is part of the port doesn't matter. I've measured Fb with corner mounted ports that use two walls as part of the port and they were the same as round ports of the same area and length mounted in the middle of the baffle.

[Adam_M]

I made a low-tuned "small" box (< 1cf) for a 10" sub w/quite a long (1.5"x10") slot - over 4ft. It was double/triple folded at the bottom of the box and came within a Hz or 2 of WinISD's "aim" (verified w/a DATS). So close that I wouldn't bother worrying about it.

Was pipe resonance an issue in this configuration? I suspect it is still above the passband of the sub, but curious as to how it sounded in the real world.

[Chris Roemer]

Pretty sure over 100-120Hz. My mains go to 40 (sometimes lower).